OBJECTIVES: 1. To further characterize the endogenous life cycle of Toxoplasma in cats. 2. To study toxoplasmosis in cats and other models for the human. 3. To study immunity to toxoplasmosis as shown by absent disease manifestations and limitation of oocyst excretion by cats. 4. To devise methods of prevention of toxoplasmosis as a disease in humans, including the production of vaccines for cats and for humans. Cats will be infected wth oocysts for study of the endogenous cycle. The nature of immunity will be studied after oral and parenteral infection, and oral challenge. A large number of strains of Toxoplasma have been collected for sero-typing and immunotyping. Strains suitable for vaccination will be considered on the basis of immunogenicity, safety and effectiveness. A vaccine will be considered on the basis of stability, effectiveness and safety. The vaccine is to be tested first in cats, which when immune would significantly reduce contamination with oocysts of the environment of man. The vaccine is to be tested subsequently in humans to afford direct protection against infection from oocysts or meat to interested exposed or vulnerable individuals. To study the endogenous life cycles of certain other coccidia of cats encountered during the studies above. BIBLIOGRAPHIC REFERENCES: Lindberg, R. E. and Frenkel, J. K. Cellular immunity to Toxoplasma and Besnoitia in hamsters: specificity and the effects of cortisol. Infect. Immun. 15:855-862, 1977. Lindberg, R. E. and Frenkel, J. K. Toxoplasmosis in nude mice. J. Parasitol. 63:219-221, 1977.